A novel non-bonded interface technique (NBIT) is used to analyze internal residual strain by combining a pre-split sample of AISI 4340 steel with the circular grid residual strain analysis technique. NBIT is compared with an implicit non-linear finite element (FE) model using LS-DYNA. A split FE model was compared with a quarter FE model to determine the split interface that causes an average difference of 9.0% on the residual von Mises strain field from a 588.6 N indentation. The homogeneous FE quarter model was then compared with the experimental split model using 588.6, 981.0, and 1471.5 N indentation forces. An average displacement difference of 3.92 µm was found when comparing the experimental split and FE homogeneous samples from a 588.6 N indentation. The internal residual major and minor principal strains from the split experimental sample and homogeneous FE model were compared for each indentation force. The minor principal strain results show the 588.6, 981.0, and 1471.5 N indentation forces resulted in a difference between the experimental split and homogeneous FE model of 28.5%, 34.8%, and 26.0%, respectively. The difference between the comparisons was explained by the inability of the FE model to simulate local non-homogeneous material properties such as grain composition and orientation whereas NBIT does. NBIT can be used for micro- or macro-scale residual strain analysis as the spatial resolution is highly adjustable.

Spring-back of poly(methyl methacrylate) (PMMA) at large strains, various embossing temperatures, and release temperatures below glass transition is quantified through modified unconfined recovery tests. Cooling, as well as large strains, is shown to reduce the amount of spring-back. Despite reducing the amount of spring-back, these experiments show that there is still a substantial amount present that needs to be accounted for in hot embossing processes. Spring-back is predicted using finite element simulations utilizing a constitutive model for the large strain stress relaxation behavior of PMMA. The model's temperature dependence is modified to account for cooling and focuses on the glass transition temperature region. Spring-back is predicted with this model, capturing the temperature and held strain dependence. Temperature assignment of the sample is found to have the largest effect on simulation accuracy. Interestingly, despite large thermal gradients in the PMMA, a uniform temperature approximation still yields reasonably accurate spring-back predictions. These experiments and simulations fill a substantial gap in knowledge of large strain recovery of PMMA under conditions normally found in hot embossing.

Nano-scale multilayer composite thin films are potential candidates for coating applications at harsh environments due to their promising mechanical and thermal properties. In this study, a viscoplasticity continuum model based on the plastic flow potential of metal/ceramic nanolayer composites, obtained from molecular dynamics (MD) simulations, is developed to build up a multiscale model bridges atomistic simulation with continuum models for the thin film composites. The model adopts a power law hardening considering confined layer slip (CLS) mechanism and accounts for the evolution of dislocation density based on the statistically stored dislocations and geometrically necessary dislocations. It is then implemented into a finite element code ( ls-dyna ) to investigate the deformation behavior of nanolayer composites at the macroscale. The deformation behavior of a high strength steel coated with Nb/NbC multilayer is also examined.

Direct numerical simulations (DNS) of knitted textile mechanical behavior are for the first time conducted on high performance computing (HPC) using both the explicit and implicit finite element analysis (FEA) to directly assess effective ways to model the behavior of such complex material systems. Yarn-level models including interyarn interactions are used as a benchmark computational problem to enable direct comparison in terms of computational efficiency between explicit and implicit methods. The need for such comparison stems from both a significant increase in the degrees-of-freedom (DOFs) with increasing size of the computational models considered as well as from memory and numerical stability issues due to the highly complex three-dimensional (3D) mechanical behavior of such 3D architectured materials. Mesh and size dependency, as well as parallelization in an HPC environment are investigated. The results demonstrate a satisfying accuracy combined with higher computational efficiency and much less memory requirements for the explicit method, which could be leveraged in modeling and design of such novel materials.

Conventional fusion joining methods, such as resistance spot welding (RSW), have been demonstrated to be ineffective for magnesium alloys. However, self-pierce riveting (SPR) has recently been shown as an attractive joining technique for lightweight metals, including magnesium alloys. While the SPR joining process has been experimentally established on magnesium alloys through trial and error, this joining process is not fully developed. As such, in this work, we explore simulation techniques for modeling the SPR process that could be used to optimize this joining method for magnesium alloys. Due to the process conditions needed to rivet the magnesium sheets, high strain rates and adiabatic heat generation are developed that require a robust material model. Thus, we employ an internal state variable (ISV) plasticity material model that captures strain-rate and temperature dependent deformation. In addition, we explore various damage modeling techniques needed to capture the piercing process observed in the joining of magnesium alloys. The simulations were performed using a two-dimensional axisymmetric model with various element deletion criterions resulting in good agreement with experimental data. The simulations results of this study show that the ISV material model is ideally suited for capturing the complex physics of the plasticity and damage observed in the SPR of magnesium alloys.

It is the objective of this study to conduct realistic simulations of the arc-height development in shot-peened Almen strips using the finite element (FE) method. Unlike our earlier work which is devoted to relaxation of shot peening induced residual stress, in this paper, the focus is on peen forming as a result of repeated spherical impingement. Specifically, a 3D FE model with 1500 randomly distributed shots bombarding an Almen strip was developed. Strain rate dependent plasticity was considered and an artificial material damping was applied to control the undesired high-frequency oscillations. The solution further adopts both explicit dynamic and implicit quasi-static analyses to simulate the entire arc-height development in the Almen strips. Quantitative relationships between the resulting equivalent plastic strain and the associated residual stress distribution for a given shot velocity and shot numbers are established and discussed. The work also considers the effect of repeated impacts upon the induced residual stress field using a large number of random shots. Attention was further devoted to the effect of the strip constraint upon the outcome of the impingement. Our results indicate that the proposed FE model is a powerful tool in investigating the underlying mechanisms of the peening treatment.

The present paper investigates the microstructure and mechanical properties’ aspects of AISI 4140 steel front axle beams developed by roll and hot-die forging processes. Microstructure of the processed beams exhibited tempered martensite, and nonmartensitic products, such as retained austenite and ferrite at the case and core, respectively. Fatigue testing results indicate that roll forged beams have demonstrated 37% higher fatigue lives (Weibull B50 life) compared to hot-die forged beams, despite similar quasi-static tensile properties. The improved fatigue performance of the roll forged beams over hot-die forged beams is attributed to the fine, close texture and rationalized material flow in the beams processed by the roll forging process. Finite element analysis and experimental strain measurements of subject component indicate that the stress levels due to fatigue loads are well below the static yield strength and endurance limit of AISI 4140 steel; however, the notches present in the form of flash or partition lines of the forged beams have initiated the fatigue failures of the beams.

In this study, a hierarchical multiscale homogenization procedure aimed at predicting the effective mechanical properties of silica/epoxy nanocomposites is presented. First, the mechanical properties of the amorphous silica nanoparticles are investigated by means of molecular dynamics (MD) simulations. At this stage, the MD modeling of three-axial tensile loading of amorphous silica is carried out to estimate the elastic properties. Second, the conventional twp phase homogenization techniques such as finite elements (FE), Mori-Tanaka (M-T), Voigt and Reuss methods are implemented to evaluate the overall mechanical properties of the silica/epoxy nanocomposite at different temperatures and at constant weight ratio of 5%. At this point, the mechanical properties of silica obtained in the first stage are used as the inputs of the reinforcing phase. Comparison of the FE and M-T results with the experimental results in a wide range of temperatures reveals fine agreement; however, the FE results are in better agreement with the experiments than those obtained by M-T approach. Additionally, the results predicted by FE and M-T methods are closer to the lower bound (Reuss), which is due to lowest surface to volume ratio of spherical particles.

High-strength low alloy steels (HSLA) have been designed to replace high-yield (HY) strength steels in naval applications involving impact loading as the latter, which contain more carbon, require complicated welding processes. The critical role of HSLA-100 steel requires achieving an accurate understanding of its behavior under dynamic loading. Accordingly, in this paper, we experimentally investigate its behavior, establish a model for its constitutive response at high-strain rates, and discuss its dynamic failure mode. The large strain and high-strain-rate mechanical constitutive behavior of high strength low alloy steel HSLA-100 is experimentally characterized over a wide range of strain rates, ranging from 10 −3 s −1 to 10 4 s −1 . The ability of HSLA-100 steel to store energy of cold work in adiabatic conditions is assessed through the direct measurement of the fraction of plastic energy converted into heat. The susceptibility of HSLA-100 steel to failure due to the formation and development of adiabatic shear bands (ASB) is investigated from two perspectives, the well-accepted failure strain criterion and the newly suggested plastic energy criterion [1]. Our experimental results show that HSLA-100 steel has apparent strain rate sensitivity at rates exceeding 3000 s −1 and has minimal ability to store energy of cold work at high deformation rate. In addition, both strain based and energy based failure criteria are effective in describing the propensity of HSLA-100 steel to dynamic failure (adiabatic shear band). Finally, we use the experimental results to determine constants for a Johnson-Cook model describing the constitutive response of HSLA-100. The implementation of this model in a commercial finite element code gives predictions capturing properly the observed experimental behavior. High-strain rate, thermomechanical processes, constitutive behavior, failure, finite elements, Kolsky bar, HSLA-100.

The low in-plane modulus of honeycombs may be used for compliant structures with a high elastic limit while maintaining a required modulus. Numerical and finite element (FE) studies for a functional design of honeycombs having a high shear strength, (τ pl *) 12 and a high shear yield strain, (γ pl *) 12 are conducted with two material selections—mild-steel (MS) and polycarbonate (PC) and five honeycomb configurations, when they are designed to be a target shear modulus, G 12 * of 6.5 MPa. A numerical study of cellular materials theory is used to explore the elastic limit of honeycombs. FE analysis is also employed to validate the numerical study. Cell wall thicknesses are found for each material to reach the target G 12 * for available cell heights with five honeycomb configurations. Both MS and PC honeycombs can be tailored to have the G 12 * of 6.5 MPa with 0.1–0.5 mm and 0.3–2.2 mm cell wall thicknesses, respectively, depending on the number of vertical stacks, N. The PC auxetic honeycomb with θ= −20 deg shows high shear flexibility, when honeycombs are designed to be the G 12 * of 6.5 MPa; a 0.72 MPa (τ pl *) 12 and a 13% (γ pl *) 12 . The authors demonstrate a functional design with cellular materials with a large design space through the control of both material and geometry to generate a shear flexible property.

This paper investigates the effect of shear stresses on the determination of residual stresses in isotropic and orthotropic materials by the slitting method. A great deal of research effort is focused on the estimation of the residual stress component normal to the slit face using strain data measured by strain gauges installed on the top or the back surface of the stressed specimens. However, the slitting process will also release two in-plane and out-of-plane shear stress components, which may influence the measured strains. For the two specimens of carbon/epoxy and glass/epoxy laminated composites as well as a steel specimen, the distribution of released strains on the top and the back surfaces due to the shear stresses is calculated using finite element method and compared with those due to the residual normal stress. The results show that on the back surface, the shear stresses have a very small effect on the measured strains. However, on the top surface, strains due to the residual shear stresses are significant compared with those due to the residual normal stress and cannot be ignored. A method using two top surface strain gauges in both sides of the slit is presented to separate the effects of normal and shear stresses from each other. Also, strains due to the in-plane and the out-of-plane shear stresses could be isolated from each other. If these separations could be carried out successfully, the residual shear stress can be calculated by the proposed formulation.

A finite element method is employed to numerically evaluate the stiffness and energy absorption properties of an architecturally hybrid composite material consisting of unidirectional and random glass fiber layers. An ls-dyna finite element model of a composite hollow square tube is developed in which the position of the random fiber layers varies through the thickness. The assessment of the stiffness and energy absorption is performed via three-point impact and longitudinal crash tests at two speeds, 15.6 m/s (35 mph) and 29.0 m/s (65 mph), and five strain rates, ɛ · = 0.1 s −1 , 1 s −1 , 10 s −1 , 20 s −1 , and 40 s −1 . It is suggested that strategic positioning of the random fiber microstructural architecture into the hybrid composite increases its specific absorption energy and, therefore, enhances its crashworthiness. The simulation data indicate that the composite structure with outer layers of unidirectional lamina followed by random fiber layers is the stiffest due to the considerable superior specific energy absorption of the random fiber micro-architecture. Moreover, it is illustrated that the specific energy absorption increases with the increased ratio of impact contact area over cross-section area. Of all the parameters tested the thickness of the unidirectional laminate on the specific energy absorption does not appear to have a significant effect at the studied thickness ratios.

Silicon carbide (SiC) is an important ceramic material usually found in polycrystalline form with grain boundary thickness ranging from a few nanometers to a few hundred nanometers and grains with multiple orientations with sizes of the order of few micrometers. The present work focuses on analyzing how the interplay between different orientations of SiC grains and different grain boundary thicknesses can be exploited for targeted improvement in the fracture resistance properties of SiC. Crack propagation simulations using the cohesive finite element method (CFEM) are performed on the finite element meshes developed on experimentally processed SiC morphologies. Analyses were performed at two different length scales: 300 μm × 60 μm (scale-1:Microscale) and 75 μm × 15 μm (scale-2:Mesoscale). Lower resolution microstructure at scale-1 does not explicitly consider the presence of grain boundaries (GBs). Higher resolution microstructure at scale-2 explicitly models GBs. Results indicate that the effect of change in grain orientation is on crack path only. The fracture resistance is not significantly affected. The presence of GBs may directly aid in strengthening a microstructure’s fracture resistance. However, indirectly it may weaken a microstructure by favoring the formation of microcracks. Significantly higher crack formation in grain interior while lower interfacial energy dissipation in comparison to interfaces indicates overall lower fracture strength of grain interiors in comparison to interfaces. If GBs are not accounted for, the second most influencing factor affecting fracture strength is the average grains size. Overall, it is mainly the GBs not the grain orientation distribution and grain size that significantly affects fracture strength.

This paper studies the effects of interfacial friction distribution on the integrity of superplastic formed parts. For that purpose, the deformation of AA5083 superplastic aluminum alloy into a long rectangular box is investigated. The die surface is divided into five regions for local application of friction coefficients. The commercial finite element code, ABAQUS TM , is used to carry out the forming simulations and calculate the thickness distribution, forming time, and forming pressure profile for different combinations of friction coefficients. It is found that friction distribution at the die-sheet interface strongly affects the metal flow during the forming process, which has a direct impact on deformation stability and strain localization. With the proposed optimal variable friction distribution, the quality of the formed parts has been enhanced, while reducing the required forming time.

Finite element analyses of oxygen diffusion at the grain level have been carried out for a polycrystalline nickel-based superalloy, aiming to quantify the oxidation damage under surface oxidation conditions at high temperature. Grain microstructures were considered explicitly in the finite element model where the grain boundary was taken as the primary path for oxygen diffusion. The model has been used to simulate natural diffusion of oxygen at temperatures between 650 ∘ C and 800 ∘ C , which are controlled by the parabolic oxidation rate and oxygen diffusivity. To study the effects of mechanical stress on oxygen diffusion, a sequentially coupled deformation-diffusion analysis was carried out for a generic specimen geometry under creep loading condition using a submodeling technique. The material constitutive behavior was described by a crystal plasticity model at the grain level and a unified viscoplasticity model at the global level, respectively. The stress-assisted oxygen diffusion was driven by the gradient of hydrostatic stress in terms of pressure factor. Heterogeneous deformation presented at the grain level imposes a great influence on oxygen diffusion at 750 ∘ C and above, leading to further penetration of oxygen into the bulk material. Increased load level and temperature enhance oxygen concentration and penetration within the material. At 700 ∘ C and below, mechanical loading seems to have negligible influence on the oxygen penetration because of the extremely low values of oxygen diffusivity and pressure factor. In the case of an existing surface microcrack, oxygen tends to accumulate around the crack tip due to the high stress level presented near the crack tip, leading to localized material embrittlement and promotion of rapid crack propagation.

Residual stresses in a stainless steel vessel containing glass have been evaluated using measurements and numerical simulation. High-level nuclear wastes are often vitrified in glass cast in cylindrical stainless steel containers. Knowledge of the internal stresses generated in both the glass and container during this process is critical to structural integrity and public safety. In this research, residual stresses were measured near the surface of a High Level Waste container using an Incremental Center Hole Drilling technique. Residual stress magnitudes were found to be at or near to the yield stress in the container wall. A transient finite-element thermal-stress model has been developed to simulate temperature, distortion, and stress during casting and cooling in a simple slice domain of both the glass and the container. Contact thermal-stress elements were employed to prevent penetration at the glass–container interface. Roughness of these contact surfaces was modeled as an equivalent air gap with temperature-dependent conductivity in the thermal model. The stress model features elastic-viscoplastic constitutive equations developed based on the temperature-dependent viscosity of the glass and elastic-plastic constitutive equations for the stainless steel. The simulation was performed using the commercial ABAQUS program with a user material subroutine. The model predictions are consistent with the residual stress measurements, and the complete thermal–mechanical behavior of the system is evaluated.

Fatigue life modeling of anisotropic materials such as directionally-solidified (DS) and single-crystal Ni-base superalloys is often complicated by the presence of notches coupled with dwells at elevated temperatures. This paper focuses on an approach for predicting low cycle fatigue that includes notch geometry effects while taking into consideration material orientation. An analytical model based on a generalization of the Neuber notch analysis to both multiaxial loading and anisotropic materials is used to determine the localized stress-inelastic strain response at the notch. The material anisotropy is captured through a multiaxial generalization of the Ramberg–Osgood relation using a Hill’s criterion. The elastic pseudo stress and pseudo strain response in the vicinity of the notch used as input in the Neuber analysis is determined from an anisotropic elastic finite element analysis. The effects of dwells at elevated temperature are captured using an equivalent strain rate. A nonlocal approach is needed to correlate the life of notched specimens to smooth specimens.

A material model for the fracturing behavior for braided composites is developed and implemented in a material subroutine for use in the commercial explicit finite element code ABAQUS . The subroutine is based on the microplane model in which the constitutive behavior is defined not in terms of stress and strain tensors and their invariants but in terms of stress and strain vectors in the material mesostructure called the “microplanes.” This is a semi-multiscale model, which captures the interactions between inelastic phenomena such as cracking, splitting, and frictional slipping occurring on planes of various orientations though not the interactions at a distance. To avoid spurious mesh sensitivity due to softening, the crack band model is adopted. Its band width, related to the material characteristic length, serves as the localization limiter. It is shown that the model can realistically predict the orthotropic elastic constants and the strength limits. More importantly, the present model can also fit the tests of size effect on the strength of notched specimens and the post-peak behavior, which have been conducted for this purpose. When used in the ABAQUS software, the model gives a realistic picture of the axial crushing of a braided tube by a divergent plug.